Semiconducting Wires Improve Polymer LEDs

European researchers have performed a balancing act that may lead to inexpensive and large-scale illumination, low-power and expansive flat panel displays, and other applications of polymer light-emitting devices. Traditionally, polymer LED design balances effective charge transport against efficient light emission. Now the researchers have developed and demonstrated a scheme that is good at both.

Researchers are investigating the use of insulated organic wires as the basis of polymer LEDs. Courtesy of Franco Cacialli.
The group accomplished this by stringing insulating beads of macrocyclic sugars, or cyclodextrins, along a polymer semiconducting wire core. Franco Cacialli, a member of the research team and a lecturer at University College London and its nascent Center for Nanotechnology, said that the wire construction began at Oxford University in the UK.

The Oxford group, he noted, directed the self-assembly of small segments of the semiconducting cores and of the insulating cyclodextrin rings using hydrophobic binding in water. A polymerization reaction coupled the core segments, and bulky groups were added at both ends to prevent the molecules from unthreading. In the final manufacturing step, team members from University College, Cambridge University in the UK and Humboldt University Berlin spin-coated the wires.

They showed that the insulated wires were photoluminescent by exciting them optically with 325-nm radiation, and they proved that they were electroluminescent by incorporating 80- to 100-nm-thick films of the molecules into LEDs. Emission was in both the blue and the green. The experiments also revealed that the insulation protected the luminescence from degradation by environmental contaminants such as metals.

Printing LEDs

The team is working to increase the light-emitting efficiency of the materials, and other areas of investigation include manufacturing schemes to improve efficiency and decrease the production costs of any future polymer LED. One intriguing possibility is the cheap fabrication of the LEDs over large areas using ink-jet printing techniques, for which these materials could be well-suited.

"Organic solvents tend to degrade the print heads more rapidly than water," Cacialli said. "Water-soluble materials such as the ones we have used for this work are thus particularly appealing."